1. A feigned claim of ill health used as a means of avoiding or absenting oneself from a political engagement of some kind. The senator's conspicuous absence from the debate was put down to the sudden contraction of a virus, but it is likely just a diplomatic flu so that he will not be forced to address recent allegations of tax fraud.
2. By extension, any claim of ill health used as an excuse to avoid an unpleasant or undesirable situation. Frustrated by her dead-end job, Janet contracted a diplomatic flu to takes some time off and reexamine her priorities.
See also: flu